It has been widely recognized that nickel, nickel alloys and cobalt alloys, among others, are very active electrocatalysts, well suited for use on a hydrogen-evolution cathode in an electrolytic cell. Among the above materials, nickel and its alloys, such as NiAl, the so called "Raney-nickel", are preferred because they are stable in a basic medium and interesting from the standpoint of availability and cost.
Raney-Nickel is prepared typically by leaching aluminum from a NiAl alloy or zinc from a NiZn alloy. The techniques developed in the past for depositing NiAl or NiZn on a substrate for producing an electrode for example, are summarized hereinafter:
(a) Deposition by a thermal-plasma technology; U.S. Pat. No. 4,240,895, granted to Olin Corporation on Dec. 23, 1980;
(b) Thermal-interdiffusion of aluminum in a nickel matrix; U.S. Pat. No. 4,116,804, granted to Dupont de Nemour and Co. on Sept. 26, 1978;
(c) Immersion of a substrate in a molten nickel alloy; A. Nidola and R. Schira, Extended Abstract No. 236 (May 6-11, 1984);
(d) Pressing at a high temperature a mixture of nickel powder and Nickel alloy powder; U.S. Pat. No. 4,278,568, granted to Metallgesellschaft Aktiengesellschaft on July 14, 1981; and
(e) Electrocodeposition of a Nickel alloy on a metallic substrate; U.S. Pat. Nos. 4,255,247 and 4,302,322, granted to Asahi Glass Company on Mar. 10, 1981 and Nov. 24, 1981, respectively.
Among the above techniques, the electrocodeposition process is the most attractive, mainly because it is relatively inexpensive to put in practice.
The electrocodeposition process may be defined as the simultaneous deposition on a substrate of two species in a solution, namely metal ions and small particles fixed on the substrate by the reduction thereon of metal ions. For the deposition of NiAl, the substrate may be placed into an electroplating nickel bath containing water soluble salts such as NiSO.sub.4, NiCl2, etc., in which is also added a certain quantity of NiAl powder, the NiAl particles being captured in the nickel matrix electrodeposited on the substrate.
With conventional electrocodeposition processes, the relative amount of co-deposited NiAl particles (hereinafter "r.sub.a ") may reach a maximum of 45 wt. % (particle size 73 .mu.m). It has been observed that r.sub.a tends to decrease significantly with time due to the passivation of NiAl particles. In addition, the maximum value of r.sub.a is significantly reduced in the presence of small particles e.g. r.sub.a is 35 wt. % for an average particle size of 30 .mu.m.
It has been widely accepted that the higher the r.sub.a value, the better the performance of the electrocatalytic electrode will be. Therefore, an object of the present invention is to provide an improved process and an apparatus for manufacturing an electrocatalytic electrode by depositing on a substrate an electrocatalytic material in particulate form, by a composite coating technique, allowing to increase the relative amount of particulate material in the coating comparatively to conventional composite-coating processes.
The process, according to a preferred embodiment of the invention, comprises the steps of placing the substrate to be coated into a metal plating bath, containing particles of electrocatalytic material such as Fe, Ni, Co or Cu, or alloys thereof, the other metal of the alloy being selected in the group consisting of Al, Zn, Cd, Sn and Mo, flowing electric current in the bath, between an anode and the substrate constituting the cathode, to induce metal deposition on the substrate by a reduction of metal ions. The cell containing the plating solution is subjected to a controlled motion causing, under the effect of gravity, a recurrent migration of the electrocatalytic material toward the substrate to build-up a coating of the electrocatalytic material, retained on the substrate in a binder formed by a reduction of metal ions on the substrate.
Advantageously, the process is used for coating, in the same operation, the two main faces of a plate-like substrate, by inverting periodically the electroplating cell so that the surfaces of the substrate are successively exposed to face up. The inversion of the electroplating cell causes electrocatalytic material, migrating toward the bottom of the cell, to adhere to the upwardly facing surface of the substrate. The inversion of the cell is repeated as many times as required in order to build up a coating of electrocatalytic material of the desired thickness on each surface.
The frequency with which the cell is inverted is a function of the particle size of the electrocatalytic material; the larger the particles, the higher the frequency because the particles migrate faster.
In addition to the cell inversion, it has been found advantageous, although not essential, to stir from time to time the plating bath, preferably before each deposion run, to induce strong interparticular collisions. The abrasive effect of such collisions has a cleaning action allowing to remove undesirable deposits from the particles surface to render them more active, thus reducing the detrimental effect of particles aging on r.sub.a.
When the particulate material is an alloy, preferably at the end of the deposition run the electrode is leached in an acid or in an alkaline medium to remove the soluble metallic component of the alloy. It is believed that this additional step increases somewhat the electrode efficiency because the small voids in the coating, resulting from the removal of the soluble metal, contribute to augment the active contact surface of the electrode.
The apparatus for carrying out the method, according to the invention, comprises an electroplating cell, receiving the substrate to be coated, connected to a suitable mechanism to impart a controlled motion to the cell for causing a migration of the particles under the effect of gravity.
Advantageously, at the bottom of the cell is provided a stirrer for agitating the solution to depassivate the particulate material as explained above.
The present invention comprises, in a general aspect, a process for manufacturing an electrocatalytic electrode, comprising the steps of:
(a) placing a substrate in a solution containing metal ions and a dispersed electrocatalytic particulate material selected in the group consisting
(1) Fe; PA2 (2) Ni; PA2 (3) Co; PA2 (4) Cu; and PA2 (5) an alloy of at least two metals, namely a first metal and a second metal, the first metal being selected in the group consisting of: PA2 (i) Fe; PA2 (ii) Ni; PA2 (iii) Co; and PA2 (iiii) Cu, PA2 ( i) Al; PA2 (ii) Zn; PA2 (iii) Cd; PA2 (iiii) Sn; and PA2 (iiiii) Mo, PA2 (1) Fe; PA2 (2) Ni; PA2 (3) Co; PA2 (4) Cu; and PA2 (5) an alloy of at least two metals, namely a first metal and a second metal, the first metal being selected in the group consisting of: PA2 (i) Fe; PA2 (ii) Ni; PA2 (iii) Co; and PA2 (iiii) Cu, PA2 (i) Al; PA2 (ii) Zn; PA2 (iii) Cd; PA2 (iiii) Sn; and PA2 (iiiii) Mo,
the second metal being selected in the group consisting of:
(b) flowing electric current in the solution between an anode and a cathode in electric contact with the solution, the substrate constituting the cathode, the current flow inducing a deposition of metal on the substrate by reduction of metal ions constituting a binder for retaining electrocatalytic material to the substrate; and
(c) causing a recurrent migration of the electrocatalytic material toward the substrate, under the effect of gravity, in order to build-up a coating of the electrocatalytic material on the substrate, the electrocatalytic material being captured in the binder.
The invention also extends to an apparatus for manufacturing an electrocatalytic electrode, the apparatus comprising:
(a) a cell for receiving a plating solution, the solution including metal ions and dispersed electrocatalytic particulate material selected in the group consisting of:
the second metal being selected in the group consisting of:
(b) an anode and a cathode in contact with the solution, the substrate constituting the cathode;
(c) an electric power source in operative relation with the anode and with the cathode to establish in the solution a flow of electric current therebetween for inducing a deposition of a metal layer on the substrate by reduction of metal ions, constituting a binder for retaining the electrocatalytic particulate material to the substrate;
(d) motive means in driving relation with the cell to impart to the cell a controlled motion for causing a recurrent migration of the dispersed particles, under the effect of gravity, toward the substrate in order to build up a coating of the electrocatalytic material on the substrate, the particles of electrocatalytic material being captured in the binder.